1. Field of the Invention
The invention is in the field of manufacturing optical elements by means of a replication process. More concretely, the invention relates to a method to produce a wafer scale package of at least two wafer-like substrates, wherein at least one comprises a plurality of optical elements. The invention further relates to a wafer scale package as such.
2. Description of Related Art
Manufacture of optical elements by replication techniques, such as embossing or molding, is known. Of special interest for mass production are wafer-scale manufacturing processes where an array of optical elements is fabricated on a disk-like structure (“wafer”) by means of replication. Subsequent to replication, this wafer structure is separated into individual optical elements (“dicing”).
Replication techniques include injection molding, roller hot embossing, flat-bed hot embossing, UV embossing. As an example, in the UV embossing process, the surface topology of a master structure is duplicated into a thin film of a UV-curable replication material such as an UV curable epoxy resin on top of a substrate. The replicated surface topology can be a refractive or a diffractive optically effective structure, or a combination of both. For replicating, a replication tool bearing a plurality of replication sections that are a negative copy of the optical structures to be manufactured is prepared, for example, from a master. The tool is then used to UV-emboss the epoxy resin. The master can be a lithographically fabricated structure in fused silica or silicon, a laser or e-beam written structure, a diamond turned structure or any other type of structure. The master may also be produced in a multi stage generation process by replication from a super master.
To achieve a cost effective mass production of replicated optical components, a wafer-scale replication process is desirable. A wafer or substrate in the meaning used in this text is a disc or a rectangular plate or a plate of any other shape of any dimensionally stable, often transparent material. The thickness is normally much smaller than the extent in the other two dimensions; this is also designated as “generally flat”. A plane of the wafer may be defined as a plane running perpendicular to the direction defined by the direction of the smallest extent of the wafer, e.g. normal to the disc or rectangular plate.
The diameter of a wafer disk is typically between 5 cm and 40 cm, for example between 10 cm and 31 cm. Often, it is cylindrical with a diameter of either 2, 4, 6, 8 or 12 inches, one inch being about 2.54 cm. The wafer thickness is for example between 0.2 mm and 10 mm, typically between 0.4 mm and 6 mm.
If light needs to travel through the wafer, the wafer is at least partially transparent. Otherwise, the wafer can be nontransparent as well. It can also be a wafer bearing electro-optical components, e.g. a silicon, GaAs, or CMOS wafer.
The wafer-scale replication allows the fabrication of several hundreds of generally identical structures with a single step, e.g. a single or double-sided UV-embossing process. The subsequent separating (dicing) step of the wafer then yields the individual optical components.
Integrated optical subsystems include functional elements, at least one of which is an optical element, stacked together along the general direction of light propagation (z-axis). Thus, light travelling along the z-axis passes through the multiple elements sequentially. These elements are integrated such that further alignment with themselves is not needed, leaving only the integrated optical subsystem to be aligned with other systems.
Integrated optical subsystems can be manufactured by stacking wafers that comprise functional, e.g. optical, elements in a well defined spatial arrangement on the wafer. Such a wafer scale package comprises at least two wafers that are stacked along the axis corresponding to the direction of the smallest wafer dimension (z-axis, axial direction) and attached to one another. One of the wafers bears optical elements and the other can comprise or can be intended to receive functional elements, such as optical or electro-optical elements. It is also possible that a second wafer does not bear any functional elements but acts as a cover or protection plate only. A plurality of integrated optical subsystems arranged side by side is formed by stacking the wafers in such a way that the optical or other functional elements are aligned. Subsequent dicing then yields the individual integrated optical subsystems.
There are different ways to attach the wafers to one another in order to achieve the wafer package. It is, for example, known to apply an adhesive layer or adhesive matrix directly in between the two wafers. Other known wafer packages, e.g. as disclosed in US 2003/0010431 or WO 2004/027880, comprise a spacer means, e.g. a plurality of separated spacers or an interconnected spacer matrix, arranged in between the two wafers. WO 2004/027880 mentions that the spacer matrix may also be part of one of the wafers.
Precise positioning of the functional elements along the z-axis, i.e. perpendicular to the plane of the wafers, is in many cases essential for the function of the integrated optical subsystem. Known wafer packages and production processes do not enable precise control of the z-distance of the functional elements that have to be aligned. For example, if only an adhesive matrix is used, it is difficult to establish a well defined thickness thereof, in particular if the optical element itself has a given extension in z-direction. Furthermore, though WO 2004/027880 controls the z-distance of the two wafers with respect to one another with the spacer means, there is no precise control of the z-position of one optical element on one wafer with respect to the other wafer or a functional element thereon, as its position with respect to its wafer may vary, especially if a replication technique is used.
It is often desired to reduce the dimensions of a wafer scale package in axial (z-) direction. The wafer itself, however, cannot be made arbitrarily thin without adversely affecting its stability.